Structural health monitoring of aerospace applications with restricted geometry

This paper presents a set of results from an experiment that is designed to evaluate a damage detection approach for through-thickness fatigue cracks emanating from a rivet hole in a high-performance aircraft bulkhead. Because fatigue cracks have been found through depot-level visual-inspections at the same location in several aircraft bulkheads, a "hotspot" approach to monitor this area with Lamb waves generated from surface-mounted lead ziconate titanate (PZT) transducers is evaluated. Detecting these fatigue cracks is challenging because the cracks propagate through an area of restricted geometry - a small plate-like area surrounded by thick webbing - which results in the interference of reflected wave components with the direct path wave components when using a pitch-catch approach. To minimize this interference, time-of-flight windows are applied to remove the reflected signals, and to increase probability of detection, Lamb wave mode tuning is used. Finally, to make the crack easier to detect, various static loads are applied to open the crack, but new challenges are presented when attempting to detect damage under a static load.